Corrosion resistant gasket for aircraft

ABSTRACT

A corrosion resistant gasket for aircraft, and more particularly to encapsulation of a conductive mesh in a pliable fluorosilicone compound that will not become bonded to mating surfaces and will also migrate upon compression during installation to the threads of attaching hardware utilized between the instrument, antenna, and aircraft structure, thereby reducing corrosion through the attaching hardware, by providing a hermetic seal around the periphery of the envelope of the attached device.

RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 09/268,456, filedMar. 16, 1999, abandoned which is a continuation-in-part application ofapplication Ser. No. 08/861,179 filed May 21, 1997, abandoned, which isa continuation-in-part of application Ser. No. 08/602,550, filed Feb.20, 1996, abandoned, which is a continuation-in-part of application,Ser. No. 08/356,983 filed Dec. 16, 1994, abandoned, which is acontinuation-in-part of application, Ser. No. 08/233,869 filed Apr. 26,1994, abandoned, which is a continuation-in-part of application Ser. No.07/932,098 filed Aug. 19, 1992, abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a corrosion resistant gasket for aircraft, andmore particularly to encapsulation of a conductive mesh in a pliableuncured, uncatalyzed fluorosilicone compound having a durometer of 40 orless that will not become bonded to mating surfaces and will alsomigrate upon compression during installation to the threads of attachinghardware utilized between the instrument, antenna, and aircraftstructure, thereby reducing corrosion through the attaching hardware, byproviding an outer bead around the periphery of the envelope to providea hermetic seal of the attached device.

Present methods of providing coupling between mating surfaces inaircraft having aluminum structures were limited to the uses of curedelastomer gaskets, metallic gaskets using sealants, or a multiple use ofcorrosion inhibitors and plating. The cured elastomer gaskets wouldallow moisture between the mating surfaces and tended to becomebonded/adhere or retain a memory under compression to the two surfacesafter a period of time and temperature cycling. The metallic gasketsalso had a permanent bonding problem due to the application of adhesivesto reduce the moisture ingress between the two surfaces. Both elastomerand metallic gaskets tended to shift the frequency of antennainstallations due to the gap they created between the two matingsurfaces, causing a shift in the VSWR of the antenna. The use of thecorrosion resistant compounds and sealants creates a time consumingprocess in application and removal and tend to crack during structureflexing, thereby allowing moisture to ingress between the matingsurfaces and causing a breakdown of the inhibitors.

Also, most gaskets presently used have a base material so dissimilar toaluminum that they thereby cause galvanic corrosion, rather than preventit, due to the fact that they cannot provide a hermetic seal bythemselves and require the use of an outside sealant which when used inhigh vibration areas or under flexing conditions tends to crack andthereby introduces an electrolyte that creates a galvanic cell.

Other alloy meshes such as Monel, a nickel plated copper alloymanufactured by The Chomerics Co. of Woburn, Mass., embedded in siliconegels are dissimilar metals with respect to aircraft structure oraircraft antenna base, such that they are subject to corrosionthemselves or cause additional corrosion through galvanic corrosion whenexposed to certain unfavorable environments. Also, it has been proven,both by lab tests and in service applications, that the use of siliconeon aircraft in areas where the silicone is either exposed to jet fuel orjet fuel vapors, the silicone deteriorates and consequently thecorrosion protection is jeopardized in the use of aircraft applications.

SUMMARY OF THE INVENTION

The present invention provides a gasket which prevents the ingress ofmoisture and or other contaminates between the surfaces of aluminumcausing corrosion or galvanic corrosion in this area. The gasket isconstructed so that it eliminates present electrical bonding methodsthrough the attaching screws and provides a positive bond through thestructure to the base of the instrument, antenna, and/or aircraft skinlap joints, electrical receptacle outlets, waste outlets, lavatoryinstallations and galley installations. Besides providing electricalbonding through surface to surface contact, eliminating all aspects ofcorrosion, the gasket is capable of reducing corrosion through theattaching hardware by migrating the insulating properties of the uncuredunvulcanized fluorosilicone material of the gasket onto the threads ofthe attaching hardware and forms an outer bead of the fluorosiliconecompound around the periphery of the installation upon compression.Although the gasket is shown in certain illustrative embodiments hereinin aircraft applications, it is also useful in marine applications wheresalt water corrodes aluminum or steel installations and wheremaintaining electrically conductive properties between mating surfacesis a serious problem.

By using the present flexible gasket comprising silver-plated strandedaluminum when low electrical bonding resistance is required in certainaircraft installations requiring less than 0.02 milliohm encapsulated inan uncured unvulcanized fluorosilicone, application time is reduced, aswell as removal and elimination of structural and component damageduring removal. Since the fluorosilicone compound provides a hermeticseal under high vibration, flexing conditions, aerodynamic conditions ofup to 0.8 Mach, and provides a seal for internal aircraft pressure ofover 30 P.S.I., there is no fear of an introduction of either anoutside, or inside introduction of an electrolyte that would create agalvanic cell. For the purposes of cost reductions in applications thatdo not require extremely low bonding resistance requirements, and onlyneed to be less than 1 milliohm, then the aluminum woven metallic meshor expanded aluminum screen can be of the same surface structure typecurrently used in the manufacture of aircraft structures. The expandedaluminum screen is favored over the metallic woven mesh for airplaneapplications. The reason for the preference of the expanded aluminumscreen for airplane applications is the pliability of the expandedaluminum which is made from an expanded metal foil is one of severalmesh (or open area) materials used in eliminating interference. It ismade from solid foil gauge metals to precision tolerances. The processinvolves a shaped tool lancing and stretching a ductile metal in onemotion. The resulting holes are diamond-shaped with a large variety ofhole sizes. Dimensions range from a {fraction (1/32)}″ in the long wayof the diamond (LWD) to approximately ½″ LWD. In shielding applicationsan LWD of ⅛″ or less is most common, although different shape holes canalso be created. Material options include selvage edge, solid sectionsand annealing. Upon compression the screen will make contact with thetwo mating surfaces to provide electrical continuity, but will not causeany pits into the airplane structure as will the woven metallic meshwith the harder durometer. This has been observed on in-service airlinefield tests and discovered after a period of two years with periodicinspections using the woven metallic mesh.

By encapsulating the woven mesh or expanded aluminum screen with afluorosilicone compound such as Dow Corning LS40 or GE FSE2120, or anyother fluorosilicone compound that is not vulcanized and catalyzed theend product can be achieved. The fluorosilicone compound is best appliedin a sheet form to a precut or die cut configuration of the metallicmesh or screen the configuration of the device to be applied to. Theexpanded aluminum screen should not exceed more than 20% of thethickness of the fluorosilicone compound. Application of the compound tothe mesh can be accomplished by placing a layer of the compound on awarm pad and placing the aluminum screen or mesh on top. Then by the useof a vacuum bag, cover the heated gasket and drawing vacuum, thefluorosilicone compound will flow through the mesh, and also fill allthe installation cut-outs for the attaching screws.

FIG. 8 shows a gasket that has an inner elastomer in the die cut for theattaching hardware that provides corrosion protection to the screws andnut plates or fasteners. FIG. 8 shows the use of two layers of thescreen or expanded aluminum mesh of FIG. 7 and die cut of the same tothe desired configuration and the insertion of the uncured elastomerbetween the two conductive layers. This eliminates the need for specialtype release liners and can be packaged for application in any type ofplastic or paper container. When applied under pressure the two aluminummesh 32 meet and form the electrical contact needed between the twomating surfaces and the elastomer 31 squeezed between the openings andouter periphery of the exposed mesh to encapsulate the mesh and providea hermetic seal. The desired thickness of elastomer 31 is approximately0.20 inch with the expanded height of the mash being 0.17 inch.Depending upon the depth of the groove or channel, the described initialbuildup can be multiple layers to form any particular height. At thesame time that the squeegee action of the elastomer is taking place, ifthere is any moisture on either of the mating surfaces, it is expelledby the application of the elastomer.

The present gasket provides a hermetic seal between two mating surfacesand provides corrosion protection between those surfaces that thepliable material is in contact with, including attaching hardware whenthe compound is migrated to the threads during installation. Theinternal mesh or screen provides a positive electrical bond to reducelightning strike and improve antenna performance such as eliminating “P”static build up when under compression of the base and structure.

In contrast to the material described e.g. in U.S. Pat. No. 4,900,877,the insulating material must be made from a fluorosilicone material thatmay be made from a commercial colloidal substance or from a thixotropicfluorosilicone material that is uncatalyzed, unvulcanized, has no watercontainment in it's make up, and free from bubbles and voids so as toprovide a hermetic seal from the outside environment as well as tocontain internal aircraft contaminates from migrating from the connectororifice to the outer edge of the instrument or antenna that the gasketis mated between. Silicone or fluorosilicone gels have a certain amountof water in their makeup. This is an undesirable feature that causesinternal corrosion by providing an electrolyte between the mesh andstructures when the gel is displaced under compression. This has beenproven during extensive in-service airplane field tests. The other notedundesirable effect was the migration of the gel and internal siliconeoils to the outer surrounding surface under compression. This siliconecontamination then reduces good paint adhesion. The conductive mesh orscreen must also have a minimum hole opening of 0.065 inch with aminimum of 8 holes per inch with the X monofilament dimension height tobe twice that of the Y dimension width so as to provide pliableconformity to the aircraft contour and provide surface contact to themating surfaces at fifteen inch pounds of compression. The conductivematerial must also be of a low resistance so as to provide the integrityneeded for system performance and against the hazards encountered withlightning strike. This precludes the use of any type of material that isidentified as E.M.I. shielding material, since these types of materialsconsist of higher resistive materials such as Monel that provide againsthigh frequency penetration or High Intensity Radiated fields (H.I.R.F.)protection.

With the present gasket, application time is minimal and removal time isequal to the application time, and further, structural and componentdamage is eliminated during removal.

The present gasket further protects the installation in harshenvironments of aircraft fluids, altitude immersions to 75,000 feet,vibration, structural flexing, and temperatures of −65° to 450° F.

DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to embodimentsthereof shown for purposes of illustration in the accompanying drawingswherein:

FIG. 1 is an exploded view of the layup of layers of a sandwichstructure used to form different geometry gaskets;

FIG. 2 is a gasket having a geometry suitable for a marker beaconantenna subsequent to assembly and die cutting of the sandwich structurelayup of FIG. 1:

FIG. 3 is an alternative die cutting of the sandwich structure layup ofFIG. 1 having a geometry suitable for an instrument gasket for mountinga total air temperature instrument to an aircraft structure matingsurface;

FIG. 4 is an exploded view of a gasket layup according to FIG. 1,utilizing a mesh in the sandwich structure;

FIG. 5 is an exploded view of a gasket layup according to FIG. 1,utilizing a knit in the sandwich structure;

FIG. 6 is illustrative of the method of assembly of an aircraft antennato an aluminum outer surface aircraft skin subsequent to removal ofrelease liners from the layup of FIG. 5 and prior to compression of theaircraft antenna to the aircraft surface.

FIG. 7 is illustrative of an expanded aluminum mesh showing strand holeopening; and

FIG. 8 is illustrative of a further embodiment of the invention where anuncured elastomer is inserted between two conductive layers.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Turning now to FIG. 1, wherein an aluminum monofilament is woven into amesh or knot electrically conductive structure 1 which is seen to bepositioned intermediate adjacent layers of compressible fluorosiliconecompound 2; electrically conductive structure 1 and adjacent layers ofcompressible fluorosilicone compound 2 are then seen to be furthersandwiched between liners 3 applied to each side of the outer surfacesof layers of compressible fluorosilicone compound 2 to protect layersand the fluorosilicone compound 2 mesh 1 structure against contaminantsprior to use. The layers 1, 2, and 3 of the sandwich assembly of FIG. 1are assembled together and die cut out into the geometry of an antennagasket 14, as shown in FIG. 2. A plurality of holes 12 are suitablepositioned in antenna gasket 14 for attaching hardware 22 utilized toattach the antenna 20 (seen in FIG. 6) to the aluminum aircraft skinsurface 30.

Gasket 33, of FIG. 3 is die cut from the sandwich assembly layup of FIG.1 to a geometry suitable for mounting an aircraft instrument to aircraftstructure. Gasket 33 includes suitable holes 35 for mounting hardware(not shown) for fastening the instrument down on gasket 33 to theaircraft structure (not shown).

Release liners 3 of FIGS. 1, 4, and 5 is of the type used to protectadhesive transfer on labels. Mesh 5 of the sandwich layup of FIG. 4 is atight weave, 0.003 in. of larger, plated with a 0.001 inch silver metalon a 0.003 mil. aluminum monofilament. In the sandwich layup of FIG. 5,instead of a mesh 4, as in FIG. 4, a knit in the form of a sock 4 isutilized, so that a spacer or non-conductive material can be inserted asshown at 40 between the outer surfaces of sock 4, so that as to make theinserted spacer or non-conductive material conductive and at the sametime prevent corrosion between two adjoining parts.

Where a weave sock 4 includes therewithin a non-conductive spacerinserted at 40 between the sides of weave sock 4 and utilized in thegasket assembly of FIG. 6 for fastening antenna 20 down to aluminumaircraft skin surface 30, a conductive corrosion-proof gasket resultswhich eliminates the previous dielectric spacer effect that reducedantenna performance, thereby providing an electrically conductivebonding between the two mating surfaces.

Prior to compression of the gasket assembly of 6 by tightening down ofmounting hardware 22 upon aircraft antenna 20 to aluminum aircraft skinsurface 30, protective release liners 3 (shown in FIG. 5) are removed,and in the compression step, fluorosilicone compound layers 2 arecompressed to result in a gasket having a thickness of the centerconductive woven silver-plated aluminum monofilament structure ofbetween about 0.31 and 0.010 inches. The present gaskets eliminate anyelectrolyte between two mating components after compression and achieveone electrical bond between the two mating surfaces. Since the gasketprovides a hermetic seal by itself, the need for additional sealantscommonly used by other types is not required with the fluorosiliconecompound or thixotropic gasket with a conductive inner mesh.

FIG. 7 shows the expanded aluminum mesh with the base materialthickness, height or “X” dimension and the strand width or “Y” dimensionin the expanded form to show the strand hole opening, or “SHO” of theelectrically conductive material 1 also known as the long way of thediamond (LWD) in commercial applications; with the fluorosiliconecompound 2; being sandwiched together with the liner 3.

FIG. 8 as hereinbefore described is similar to the embodiment of FIG. 7with the deletion of the liner and the addition of a further aluminummesh.

What is claimed is:
 1. A method of electrically conductively bondingopposing mating surfaces comprising the step of: providing a layer ofuncured, uncatalyized thixotropic fluorosilicone; disposing anelectrically soft compressable conductive woven member between saidlayer of fluorosilicone; then compressing the opposing mating surfacestogether, thereby electrically conductively bonding said opposing matingsurfaces together.
 2. A method as described by claim 1 that compensatesfor high vibration damping and allows for airplane flexure withoutcausing structure damage, known as “dimpling” or “fretting” found incurrent gel type gaskets containing non compressable conductive members.3. A method of electrically conductively bonding opposing matingsurfaces comprising the steps of: providing a layer of fluorosiliconecompound; dispersing an electrically conductive woven member betweensaid layer of an uncured, uncatylized, unvolcanized fluorosiliconecompound, containing no discernible silicone oil or water in thecompound; inserting said conductive member pair of layers between saidfluorosilicone compound between the opposing mating surfaces; and, thencompressing the opposing mating surfaces together, thereby electricallyconductively bonding said opposing mating surfaces together andproviding a hermetic seal, that is resistant to hydrocarbons found inmodern day airplanes.
 4. A method as described by claim 3, that containsno discernable oil, water, or other fluids in the compound that will notdisperse from the conductive layer under airplane flexure and highvibration and will not permit voids to allow inner or outer fluidpenetration thereby causing corrosion.
 5. A method of electricallyconductively bonding opposing mating surfaces comprising the step of:providing a of layer of thixotropic compound; disposing an electricallyconductive woven member of thixotropic compound; placing said conductivewoven member between the opposing mating surfaces of the uncured,uncatalyized thixotropic fluorosilicone; and then compressing theopposing mating surfaces together, thereby electrically conductivelybonding said opposing mating surfaces together.
 6. A method as describedby claim 5 that eliminates the use of special protective silicone coatedrelease liners required by gel type gaskets having the gel exposed tothe mating surfaces being applied to.